I think someone has to be very selfish and entitled to decide who your partner may or not befriend... Relationships are about trust, if you can't trust the person you are with why are you with him/her?
If someone wants to cheat there are many ways and opportunities...you can't control always what your partner do but you can hope the trust you have put on your partner is deserved and living with the fear of being betrayed will make you as unhappy as actually being betrayed.
I wanted to tell her everything, maybe if I'd been able to, we could have lived differently, maybe I'd be there with her now instead of here. Maybe... that would have made the impossible possible. Maybe, but I couldn't do it, I had buried too much too deeply inside me. And here I am, instead of there.